Abstract Psychiatric disorders such as major depressive disorder (MDD) are complex diseases where negative stressors increase the likelihood of its onset through epigenetic changes, such as DNA methylation (DNAm). Presently, disease-associated DNAm patterns of MDD have not been fully elucidated. This is in part due to the reliance of peripheral tissues such as blood, buccal cells, and saliva where it is unknown how DNAm changes from these tissues might be an accurate reflection of those changes in the brain. As such, there is an urgent need to provide a comprehensive database that would cross-compare DNAm patterns between the brain and other peripheral tissues to highlight candidate regions where disease-associated DNAm can be observed. To this end, our overall goal is to establish a comprehensive epigenomic database where DNAm patterns can be cross-compared between key brain regions (e.g., the hippocampus, amygdala, and dorsolateral prefrontal cortex), their neurons and glia components, and peripheral tissues. Our central hypothesis is that disease- associated DNAm will more likely reside in regions that vary in DNAm between different tissues. Our specific aims will address the following questions: In Aim 1, DNAm will be evaluated at a genome-wide level and cross- compared between brain and peripheral tissues to identify regions which have varying DNAm between tissues. In Aim 2, we will expand DNAm assessment in neuron and glia to the aforementioned key brain regions involved in MDD and compare that with peripheral tissues. In Aim 3, disease-associated DNAm regions will be evaluated using pathology-archived brain tissues from 206 MDD cases and 206 matched controls. This is the largest post-mortem study of MDD that specifically evaluates DNAm. Disease-associated DNAm will be overlaid with tissue-specific DNAm to determine regions where MDD-associated DNAm patterns will likely to occur. This proposed study is significant as it will provide a comprehensive epigenetic database that will guide researchers to candidate regions and suitable peripheral tissues to investigate DNAm changes in MDD. Its use may also be extended to other psychiatric disorders. This proposed study is innovative as it uses fresh brain, neuron, glia and peripheral tissues from the same individual to establish an epigenome database. It also uses disease-associated DNAm identified from a large cohort of post-mortem MDD brains to identify regions that are ?hot-spots? for disease-associated methylation changes.